1. Field of the Invention
The invention relates to a hydraulic drive, in particular for a hydraulic press. The invention moreover relates to a method to operate a hydraulic drive.
2. Description of the Related Art
Hydraulic drives are widely known from the current state of the art. In practice, it is desirable for hydraulic drives, in particular for hydraulic drives for hydraulic presses to provide a hydraulic drive that, on the one hand provides a rapid movement of a drive piston in a so-called rapid stroke or rapid movement with low force and with which on the other hand a slower action with great force is possible in a so-called load stroke or load movement.
Various drives are known for this purpose from the current state of the art. In one drive with a so-called throttle control, the control and changeover between rapid stroke and load stroke through control of the volume flow occurs hereby via flow resistances between the pressure supply and cylinder. A disadvantage of such a drive with throttle control is the low efficiency due to the occurring flow losses.
Drives having a so-called displacement control system are moreover known from the current state of the art. A drive of this type may for example comprise a variable speed motor that drives two pumps having opposite delivering directions. The two pumps are connected via a hydraulic cylinder in such a way that the pump takes in hydraulic oil from one piston chamber of a hydraulic cylinder, whereas it moves hydraulic oil into the other piston chamber. The changeover from rapid stroke to load stroke, or respectively the speed control of the hydraulic drive occurs thereby through changing of the displacement volume of the pump or respectively through a change in the rotational speed of the motor. A variable displacement pump with changeable displacement volumes is expensive and noisy. Further, when using a pump having a constant displacement volume a changeover from a rapid stroke to a load stroke is not at all possible. A disadvantage of such a drive with a displacement control system without rapid and load stroke is that the motor must have a higher speed for the high speed in the rapid stroke, whereas a high maximum torque is required for the high force in the load stroke mode. Therefore, because of this high so-called peak performance the hydraulic drive becomes large, heavy, slow and expensive.
What is needed in the art is a hydraulic drive that can be operated with lower efficiency losses in a rapid stroke and a load stroke mode and whereby the motor should be able to be produced cost effectively.